This invention relates to superconducting magnet assembly for a magnetic resonance imaging system (hereinafter called "MRI"), and more particularly to an improved and simplified arrangement for improving magnetic field homogeneity in such an assembly.
As is well known, a superconducting magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold ensures that the magnet coils are made superconducting, such that when a power source is initially connected to the coil (for a period, for example, of up to one hour) to introduce a current flow through the coils, the current will continue to flow through the coils even after power is removed due to the absence of resistance, thereby maintaining a strong magnetic field. Superconducting magnets find wide application in the field of MRI.
However, MRI requires very strong or large magnetic fields in the imaging bore with a very high degree of uniformity or homogeneity. Typically this homogeneity requirement, on the order of 10 parts per million (ppm) on 40 to 50 cm diameter spherical volume (DSV), cannot be achieved by controlling manufacturing tolerances. In practice shim systems which may be extra coils, typically called correction coils, small pieces of iron, typically called passive shims, or some combination of the two are provided to correct or improve the magnetic field homogeneity. This shim system allows reasonable manufacturing tolerances. Typical magnet homogeneities after magnet manufacture are on the order of a few hundred ppm. Use of such shim systems can reduce this homogeneity to the required 10-15 ppm.
Considerable effort has been directed at systems which provide the required homogeneity yet which are uncomplex in structure and adjustment. Current correction coil designs utilize correction coil formers or supports, typically of fiber reinforced epoxy plastic (FRP) material. Wire coils are wound on the correction coil former with the correction coil system consisting of a plurality of correction coils added to the interior of the superconducting magnet assembly. These correction coils are typically designed to create single harmonics as purely as possible. One axial correction coil system has 14 coils to create or provide "6 orders of shim". In addition, a transverse correction coil system is typically added to the shim system. Such systems are complex and costly and require considerable space in the MRI magnet assembly.
Current passive shim systems also utilize another FRP drum that is inserted in the warm space inside the magnet bore. Shims or pieces of magnetic material are mounted on drawers that slide into slots on the drum. The drawers must be removed and reinserted into the drum while the magnet is at field (in superconducting operation). However, the strong magnetic field created by the superconducting main magnet coils exert forces on the drawers and on the internal correction coils. In the case of, for example, a 1.5 Tesla magnet these forces can be very large. In addition, as the ambient temperature surrounding the passive shims increases, the homogeneity of the magnet is degraded. Minimizing the total amount of shim will reduce this undesirable effect.
One design of such shim drums is shown in U.S. Pat. No. 5,389,909 of Timothy J. Havens, entitled "Open Architecture Magnetic Resonance Imaging Passively Shimmed Superconducting Magnet Assembly", which is assigned to the same assignee as the present invention.
Such existing shimming systems tend to be relatively complex, costly and difficult and time consuming to adjust to obtain the required field homogeneity. The structure provided to support the shims must be mechanically strong enough and rigid enough to resist the strong magnetic forces exerted on the shimming structures by the strong main magnetic field. In addition, such support structures add complexity and weight to the superconducting magnet. Still further, passive correction systems tend to be large and transverse correction coils are costly such that a more efficient overall field homogeneity system also lowers cost and size.